In real FLIR systems, an infrared sensor or detector scans a field of view and produces from this scanning a video image that displays to the user the amount of detected radiation. Generally, the FLIR display is monochrome and the intensity of the monochrome display at each point of its field of view is a function of the temperature and surface characteristics of the object viewed at that point, the distance of the object from the sensor and atmospheric absorption characteristics, with white either representing hot or cold, at the election of the designer or user, and black representing the opposite. Alternatively, the FLIR output can be color video, wherein the color of each point of the display is a function of the temperature of the object viewed at that point. In either case, however, the video output to the viewer is dependent upon the detected radiation, which is indicative of the temperature of the object emitting it. The raw output of the infrared sensor of a real FLIR is normally AC coupled, either based on the frame or line of the display, and is displayed as video on a monitor, such as a CRT screen, or via another display device, such as a head mounted display projecting the image to the eye of the user, or a head-up display.
Depending on the conditions and the temperatures of objects viewed with the FLIR, the detected radiation may indicate temperatures that range over more than a thousand or over only a few degrees Celsius. Generally, the temperatures seen in a FLIR are attenuated by the distance from the sensor to the object and the atmospheric conditions, especially humidity, and, where there is attenuation, the range of temperatures detected by the sensor is compressed toward the temperature of the ambient atmosphere. This means, for example, that an object with a temperature of 40° C. viewed at a distance in air with a temperature of 20° C. might appear in a real FLIR to have a temperature of only 30° C. or even as little as 20.1° C. or less, depending on visibility or atmospheric conditions.
To display the FLIR sensor view to the user, the AC-coupled sensor output is subjected to an adjustable gain that varies the resolution of the temperatures displayed. When the gain is turned down, the range of temperatures represented by the range of monochrome shades of gray from white to black is relatively large, e.g., hundreds of degrees. This wide temperature range means that there is low temperature resolution, i.e., that temperatures a few degrees apart are all displayed as about the same monochrome shade of grey, making it difficult to distinguish between objects whose radiant energies are relatively close to each other. This is especially a problem where there is IR attenuation due to low visibility, high humidity, rain soaking, etc., and all objects tend to appear in infrared to be within a few degrees of ambient.
To overcome this, in real FLIR systems, the user can increase gain by a manual control, or an automatic gain control (AGC) may be provided so that the best level of viewability is provided on the display. When the user (or the AGC) increases gain, the range of temperatures displayed between white and black is attenuated to a narrower range, usually centered on the ambient temperature. Temperatures outside this range are displayed as either black or white, depending on whether they are hotter or colder than the operative temperature range, an effect referred to as “clamping”, but temperatures within the range are in higher temperature resolution and more readily distinguishable from each other, because an equivalent difference in the monochrome gray shade signifies a smaller temperature difference at the higher gain.
Simulation of such a display presents substantial difficulties due mainly to the difference between the AC coupled IR sensor data compared with normal video produced by image generators (IGs) generally used in simulators. Image generators used in aircraft or vehicle simulators generally are directed to creating imagery, like out-the-window scenes, that are simply to be viewed by a user as realistic imagery without added processing. IGs therefore generally do their calculations using variables of limited bit-size adequate to make common video imagery, and they calculate the color and shade of imagery and normally generate video output using 3×8-bit color values (8 bits for each of the red, blue and green color channels for each pixel) which is adequate color differentiation for realistic imagery that is projected for human viewing. This format is somewhat limiting in terms of output of a monochrome image, however, since, with 3×8-bit video output, the video output of the image generator is limited to 255 shades of gray.
This form of output is especially dissimilar to the AC-coupled output of a real FLIR sensor when it comes to the application of gain. A real FLIR output, on increase of gain, has a full range of gray scale values, and fine differences between temperatures near ambient are magnified as gain is turned up, yielding a higher resolution display. In contrast, in a 3×8-bit pixel video output, an increase in gain would only produce a reduction in the number of shades of gray, with clamping of the temperatures outside the narrowed range, and there could be no higher resolution of close temperature differences, because that information is not present in the video output.
Some prior art systems have tried to provide higher resolution of the simulated FLIR output by using larger numbers of bits in the internal processing of the imagery and in the output video. These systems theoretically provide adequate output to satisfy the necessary dynamic range of the FLIR display. However, calculation with this larger number of bits comes at a cost premium, and the requirement of color calculation with larger bit sizes greatly restricts the choices as to which of the available image generators can be used in the simulation.